Lightning arrester



BEST AVAILABLE COP" June 27, 1944. R. H. EARLE ETAL 22,504

LIGHTNING ARRES'IER Original Filed April 4, 1936 5 SheetS -Sheet lIIIVIIIIIII ffaip/z H ff arl' 0971/02 GT Stein/72a 0/" Wdm42m m,

June 27, 1944.

R. H. EARLE ETAL LIGHTNING ARRESTER 3 Shee ts-Sheet 2 Original FiledApril 4, 1936 72.7156 02, microseconds 0 N7 4 2 I A I .1 I -36 4Z2 l Z Ifl'nzc 12 mc'cmseconds 'a-J E'UJw BEST AVAILABLE COP June 27, 1944. R.H. EARLE ET AL 22504 LIGHTNING ARRESTER I Original Filed April 4, 1936 5Sheets-Sheet I5 Z 42/ Q BLT/$255562 Z4561 l 1 {if/ K 55 i l i I l 1 1 lL I l 1 1 Reissued June 27, 1944 BEST AVAiL-ABLE COP OFFICE 22,504LIGHTNING ARRESTER Ralph H. Earle, Wauwatosa, and Alwirm G. Steinmayer,Milwaukee, Wis., assig'nors to Line Material Company, South Milwaukee,Wis., a corporation of Delaware Original No. 2,165,964, dated July 11,1939, Serial No. 72,734, April 4, 1936, which is a continuation ofSerial No. 716,244, March 19, 1934. Application for reissue January 1'7,1944, Serial 3 Claims.

Our invention relates generally to lightning arresters and it hasparticular relation to lightning arresters for protecting transmissionlines operating at a frequency of 60 cycles or less.

This application is a continuation of our application Serial No.716,244, filed March 19, 1934.

In order to protect an electrical power transmission system fromabnormal voltages due to lightning and switching disturbances, surgearresters are provided. The arresters are connected between thetransmission line conductors and ground in order to drain off the excessvoltages caused by these disturbances. The ideal arrester willimmediately relieve the system of any abnormal voltage by providing aconducting path to ground through which current can flow but which willnot permit'the flow of power current under normal operating conditionsat normal line voltage. That is, during normal operating conditions thearrester offers a substantially infinite resistance to the flow of powercurrent at the system voltage but on the occurrence of an abnormalvoltage caused by lightning or switching disturbances, the resistance ofthe arrester becomes such that it provides a path to ground throughwhich the surge current can flow until the voltage causing it has beendissipated, at which time, however, power current is not permitted toflow.

A typical lightning arrester for distribution systems comprises a sparkgap assembly arranged to be connected in series circuit relation withvalve material that offers a comparatively high resistance to the flowof current. An arrester, comprising the series connected spark gapassembly and valve material, is connected between each of thetransmission conductors and ground. The valve material may comprisesilicon carbide in various forms, either granular or in blocks, or itmay comprise lead peroxide pellets which, upon heating, become coated attheir contact points with litharge. Various other types of valvematerial may also be used.

It has been customary in the construction of lightning arresters in theprior art to provide them with an opaque housing of ceramic materialsuch as porcelain around the spark gap assembly and valve material. Thehousing serves as a frame for supporting the arrester element as aunitary structure and permits mounting of the arrester on a pole ortransmission tower for connection to a transmission conductor.

On flow of surge current, due either to alightning stroke or to aswitching disturbance, a conducting path is momentarily establishedthrough the arrester which, in the ideal arrester, as statedhereinbefore, should be disrupted on termination of flow of surgecurrent. In this manner the arrester simulates the action of a valvewhich is opened on occurrence of an abnormal voltage on the system topermit the flow of surge current to ground but which is automaticallyclosed as soon as the excessvoltage isdissipated. -If the valve does'notclose or the conducting path is not disrupted, then power current ofnormal line frequency may flow, causing an undesirable leak or powerloss and also destroying the eiiectiv'eness of the arrester forprotecting the system against flow of power current after a succeedingabnormal voltage condition.

The flow of surge current through the arrester due to abnormal voltagecaused by a lightning or a switching disturbance takes place only duringan infinitesimally short time. Ordinarily, the surge current will flowonly during a few microseconds. However, the amount of current whichflows may be relatively great and in many cases it is enough to destroythe arrester entirely or to crack the porcelain housing. In such casethe customary casual inspection will reveal the failure of the arresterand it may be replaced with a new one.

In many instances, however, the flow of surge current is not enough tototally or even partially destroy the arrester or its housing, but itmay be sui'licient to fuse some of the particles, forming the valvematerial, into a conducting path of comparatively low resistance to flowof power current. After the flow Of surge current has ceased, theresistance of the arrester is not restored to its normal orsubstantially infinite resistance to flow of current at power frequency,and a slight discharge or flow of power current takes place through it.When an arrester is operating under these conditions, it should bereplaced, since it no longer is capable of interrupting the flow ofpower current after a surge breaks down the gap. The next surge maycause the arrester to become fully grounded, thereby possibly causing ashort circuit on the system, necessitating the operation of circuitbreakers or fuses to clear the fault.

The continued flow of power current through the path rendered conductingby the surge current and across the spark gaps, causes the arrester tofunction somewhat in the manner of a spark gap radio transmitter. Thetransmission line acts in this case as the antenna of a radiotransmitter. Although the radiating power of such a transmitter may becomparatively slight,

BEST AVAlLABLE COP 2 still considerable interference with radioreception results, particularly in those instances where radio receiversare operated in the vicinity of the power line. The radio interferencemay be sufficiently severeto'totally prevent the reception of anybroadcast program or radio communication.

When arresters of the prior art are employed using opaque housings ofporcelain or the like, it is not possible to determine whether or notthey have failed to this extent without a careful examination andtesting of each arrester. This means that a periodic examination of thearresters must be made and a test conducted to determine theirresistance or a succeeding surge must be awaited to completely destroythe arrester or cause it to ground the system. Even when the arresterhas been grounded, unless it is fractured in some manner by the flowthere through of surge or power current, it may be difficult toascertainthe exact location of the fault. Under such conditions it maybe necessary to disconnect a largenumber of arresters which are inproper operating condition in order to isolate the particular arresterthat has failed.

The object of our invention, generally stated, is to provide a surgearrester for transmission lines of commercial frequency which shall besimple and efficient in operation and which may be readily andeconomically manufactured, installed and inspected.

The principal object of our invention is to provide for visuallyindicating that a surge arrester on a power line has failed.

An important object of our invention is to pro vide for visuallyindicating the flow of power current through a path in a lightningarrester rendered conducting by a lightning discharge.

Still another object of our invention is to provide for renderingvisible from any point the light emitted on flow of power current from apath in a lightning arrester rendered conducting by a lightningdischarge.

A more specific object of our invention is to provide a lightningarrester comprising spark gap and valve material assemblies with ahousing of light transmitting material, whereby the light emitted onflow of power current through a path in the valve material renderedconducting by a lightning discharge, will be visible.

Other objects of our invention will in part be obvious and in partappear hereinafter.

Our invention accordingly is disclosed in the embodiments hereof shownin the accompanying drawings and it comprises the features ofconstruction, combination of elements and arrangement of parts whichwill be exemplified in the constructions hereinafter set forth, and thescope of the application of which will be indicated in the appendedclaims.

For a more complete understanding of the na ture and scope of ourinvention, reference may be had to the following detailed description,taken in connection with the accompanying drawings, in which Figures 1,2 and 3 illustrate, diagrammatically, different circuit connections forlightning arresters as applied to commercial power distribution systems;

Figures a and 5 show curves which illustrate certain operatingcharacteristics of a power distribution system on application thereto oflightnlng or switching disturbances;

Figure 6 is a view, in side elevation, of a lightning arresterconstructed in accordance with our invention;

Figure 7 is a sectional view showing the interior details ofconstruction .ofour novel lightning arrester; i 1 l Figure 8 is adiagrammatic view showing different operating conditions of a lightningarrester on flow therethrough of surge current; and

Figures 9, 10 and 11 are views showing different types of faults whichmay occur in lightning arresters as a result of the application to atransmission system ofulightning or switching surges. As a result of acareful study of the results of lightning and switching disturbances ona power system having connected thereto lightning arresters for drainingoff the abnormal voltages,

we have discovered that the discharge takes place in the arrester, inmost instances, along the inner periphery of the housing surrounding thespark gap ass emb1y and valve material. That is, on the occurrence ofaacomparatively light surge, the'discharge takes place, through thearrester and, on termination of flow of surge current, the conditionof-thearrester is, restored to its normal condition to offer a.-substantially infinite resistance tothe flow of powercurrenttherethrough. ,On the occurrence of an exceptionally heavy or extremelyhigh surge voltage, a surge current of such appreciable value may flowthrough the arrester that it is-destroyed or the housing is cracked,s0.,tha t the arrester is effectually destroyed A large number of surgesoccur, however, between these two values, the current in each of whichissufficient to damage the arrester but is'not great enough to destroyit or even to crack the housing so as to entirely clear the line of thearrestely We haveobserved that it isthis type of discharge which takesplace generally along the inner wall or periphery of the housingsurrounding thespark gap assembly and valve material. -This.discharge issuflicient, in many instances, to fusetogether a number ofthe particlesforming thevalve material, so that a path of comparatively lowresistance is formed in the arrester, thereby lowering its resistanceand permitting the flow therethrough of a slight amount of 1 powercurrent at. power frequency from the transmission ,line.

Since the resistance of the arrester under these conditions, stillremains at a comparatively high value, only asmall amount of powercurrent will fiow therethrough. However,, this amount of currentconstitutes a leak from the power system and, in flowing between theelectrodes of the spark gap assembly, generates oscillations of radiofrequency which tend to interfere with radio reception in the vicinityof the power line. A more serious objection, however, is that theeffectiveness o'f-the arrester for protecting the transmissionline fromoutages caused by the flow of powercurrent after static surges passthrough the-arrester, is largely destroyed. Therefore, an arrester whichis operating under these conditions should at once be removed from thesystemand replaced with a new arrester in proper operating condition.

While it is comparatively easy to inspect a transmission line and notearresters that have completely failed due to the excessive flow ofcurrent therethrough causing.- their destruction, it is difficult, ifnot; impossible, ,to detect the arrester that has only partially failedand which constitutes a slight leak in the insulation of thetransmission line from ground. This is due to the fact that it has beenthe practice in the past seer AVAILAQHE. 90E;

house the spark gap and valve material as-' .emblies in an opaquehousing of porcelain or ;he like through which it is, of course,impossible to see from the exterior, and likewise through which it isimpossible for light rays to be transmitted, caused by current flowingalong paths in the arrester that have been rendered conducting bylightning or switching surges.

In order to provide for readily inspecting lightning arresters asinstalled on a transmission line without making a detailed examinationand test, we have dispensed with the customary opaque porcelain housingand have substituted therefor a housing of relatively thicklight-transmitting material such as glass, and preferably of aheatresisting type which may be purchased in the open market under thename of Pyrex. The. commercial; product known as Pyrex has a very lowcoefficient of expansion and a high melting point. Since the how ofpower current along a path rendered conducting by a lightning dischargegenerally takes place along the inner surface of the glass housing,light rays emanating therefrom are readily visible through the glasshousing and, as a result, it is a simple matter to determine byinspection, even when the observer is some distance away from thearrester, that it has failed and that it should be replaced. Moreover,the housing of glass is made comparatively thick, so that light raysemanating from along a path at one side of the arrester will be visiblefrom any point thereabout, due to the reflecting characteristics of theglass wall of the housing.

In order to illustrate in more detail the various applications of ournovel lightning arrester to power systems of 60 cycles or less, certaincircuits are illustrated in the drawings. These circuits are typical ofthe distribution circuits which it is the present day practice to usefor distributing electric power for domestic and industrial purposes.

Referring now particularly to Figure 1 of the drawings, it will beobserved that a 3-phase transformer, shown generally at I5 is provided,having a delta connected primary winding I6 which may be connected forenergization to a source I! of alternating current, such as a 60 cyclegenerator. The transformer I5 is provided with a star connectedsecondary winding I8, the terminals of which may be connected throughdisconnecting switches I9 to energize transmission line conductors A, Band C. The neutral point 20 of the secondary winding I 8 is grounded at2|, as illustrated. Since each phase of the secondary winding 18 isarranged to have 2300 volts impressed thereacross, a voltage of 4000volts appears between each of the conductors A, B and C, as illustrated.

In order to provide for connecting the high voltage conductors A, B andC to a secondary distribution system for domestic or industrial use, theprimary winding 22 of a distribution transformer, shown generally at 23,is connected across the conductors B and C. The secondary winding 24 ofthe distribution transformer 23 is provided with amid-tap so that athree-wire 110 volt circuit is provided, as is customary for residentialuse.

In order to protect the distribution transformer 23 from surge currentdue to lightning and switching disturbances, lightning arresters,

shown generally at 25, are provided. Each of the arresters 25 comprisesa series of spark gaps 26 connected to a resistor 21 having acomparatively high resistance such as valve material comprising silicomcarbide or the like. The arresters 25 may be grounded at 28 and 29.

If a lightning or switching surge occurs on the conductor C it will passthrough the arrester 25 to ground at 28, breaking down the spark gaps26- and causing current to flow in an amount corresponding to thevoltage accompanying the disturbance. It will be noted that in thisinstance a line voltage of 2300 volts isapplied across thev arrester 25between the conductor C and ground. 28, which is, of course,electrically connected to the ground 2| and the neutral point .20 of thesecondary winding 18. If the amount of surge current is such that thearrester25 is capable of handling it without failure or damage thereto,the arrester will interrupt the follow current when the next zero pointin the wave of power current occurs and the systemwill be restored tonormal operating conditions.

Asa modification of the circuit connections; shown in Figure 1, thecircuit connectionsshown in Figure 2 may be used. It will be here notedthat a neutral or ground conductor G is provided which is connected tothe neutral point 20 of the secondary winding l8 of the transformer l5.This conductor G is carried along with the conductors A, B and C,forming the transmission line, and it is grounded at various points,30and 3!, for example. In this instance the primary winding 22 of thedistribution transformer 23 is connected between the conductor C and theground or neutral conductor. G and it has ap-. plied thereto 2300 volts.If a lightningor switch ing surge strikes the conductor C the. dischargethrough the arrester 25 to ground 28 will take place as describedhereinbefore in :connection with Figure 1. However, if the surge shouldstrike the ground or neutral wireG, it may pass to ground through one ofthe ground connec-- tions at 30 or 3|, or if there is no groundconnection close enough, the surge will travel along the neutral wire Guntil it comes to a lightning arrester which may be connected thereto,at which point it will pass to ground, Since the conductor G is atground potential, .there will be no follow current and, as a result, theonly damage that can possibly be done results from the flow of surgecurrent and not from the how of follow current.

Still another form of transmission circuit which may be employed isillustrated in Figure. 3 of the drawings. As there shown, the transeformer I5 is provided having its secondary winding [8 connected in deltato the conductors -A, B and 0. Each phase of the secondary winding 18may have applied thereto, as indicated, 4800 volts and this voltage isapplied to the primary winding 22 of the distribution transformer 23connected across the conductors B and C. In the event that a lightningor switching surge appears on one or the other of the conductors B or C,it will be dissipated through either of the arresters 25 to the groundconnections 23 or 29, respectively. Since the secondary winding I8 ofthe transiormer I5 is not grounded, there will be no follow current andthe only damage that can occur, providing the remaining conductor A ofthe power line is ungrounded, will be due to the flow of surge current.It often happens, however, that an accidental ground, as at 32, occurson one of the conductors, (here shown, 'the conductor A), and then thefull line voltage of 4800 volts will be efiective to continue the flowof power current along the path which has been aEsT AVAIL/isle dog; 1

rendered conducting by the flow of surge current due to the occurrenceof the abnormal or excessive voltage resulting from the lightning orswitching disturbance.

' From a consideration of the foregoing description of varioustransmission circuits which are now. in use, it will be observed thatthe flow of power current constitutes an important factor in theapplication, construction and operation of lightning arresters. If onlythe surge current had to be considered, then the problem would beconsiderably simplified. However, the path rendered conducting by theflow of surge current, either as a continuous conducting path throughcontiguous particles, or through an ionized atmosphere, instantly offersa path for the flow of power current due to the ever present linevoltage which tends to maintain this current flow. It will, therefore,be clear that the flow of surge current resulting from lightning orswitching disturbances, prepares the way for the flow of power currentthat may or may not take place, depending upon the amount of current inthe surge, the time of its occurrence, and the conditions created in thearrester as a result of its flow therethrough.

In Figure 4 of the drawings, units of amp-eres of current fiow, during alightning discharge, are plotted as ordinates, while units of time, in

microseconds, are plotted as abscissae, for the curve3B. The curve 35illustrates a typical relationship between the amperes of surge currentand the time of duration of the surge, which, as illustrated, may be ofthe order of 20 microseconds. The curve 31, shown in Figure 5 of thedrawings, is plotted with units of volts as ordinates and units of timein microseconds as abscissae, for one cycle of a BO-cycle alternatingcurrent. It will be observed that each half cycle corresponds toapproximately 8333 microseconds and therefore that the time during whichthe lightning surge takes place, as indicated by the curve 36 in Figure4 of the drawings, when plotted to the scale-used for the curve 3?,would be only thewidth of a very thin line. If the lightning dischargetakes place at the time :r, then the follow current, as represented bythe balance of the curve 37, to its next zero point, will tend to flowfor a time corresponding to the balance of the curve 3'! before itpasses through its next zero point, assuming unity power factor. If thesurge occurs at the time y, then the time during which follow currenttends to flow will be correspondingly decreased. If the surge shouldoccur at the time a, corresponding to the zero point of the voltage wave31, then there would be no tendency for flow of follow current unlessthe surge current was of such a magnitude that the arrester fails andtherefore conducts power current during the next half-cycle It will thenbe observed that the likelihood of the flow of power current during atleast one half-cycle is comparatively great and that it is the flow ofpower current over the path which has been rendered conducting by thesurge current that constitutes one of the principal factors in theoperation of a lightning arrester. If the surge would always occur at atime corresponding to the Zero point of the voltage wave 37 or at thetime 2, then the damage to the arrester caused by the flow of followcurrent would ordinarily not occur. However, such is not the case undernormal operating conditions, and there is always the possibility that acertain amount of follow current will flow for a large portion of a halfcycle at least, during which time a sumciently low resistance path maybe created and maintained in the arrester so that during the nextsucceeding half cycle the power current will continue to flow, althoughnot in an amount which would be sufiicient to cause destruction of thearrester. It is to give an indication of the flow of such current thatour invention 'is particularly useful and has provided a highlysatisfactory means whereby the flow of such current can be readilydetected with a minimum of inspection.

Referring now particularly to Figure 6 of the drawings, it will beobserved that a lightning arrester, shown generally at 4|, is thereillustrated, which is provided with a conductor 42 for connection to oneof the power conductors of a transmission line and a terminal nut 43 towhich a ground conductor may be connected by means of a terminalassembly 44. The arrester M is provided with a housing 45 which isformed preferably of a heat resisting glass that may be purchased in theopen market under the name of Pyrex. The outer surface of the glasshousing ()5 is provided with beads 46 extending entirely or partlyaround it in order to increase the creepage distance between the top andbottom portions of the housing and also to increase the light dispersiveefiects of the glass housing on flow of power current through a pathrendered conducting in the arrester by surge current. A cap @7, formedpreferably of insulating material, :uch as glass, is positioned on topof the arrester The interior details of construction of the arrester 4|are more clearly shown in Figure '7 of the drawings. As thereillustrated, a spark gap assembly, shown generally at 49, is providedwhich may comprise four cylindrical spark gap electrodes mounted betweeninsulating posts 5! which are provided with caps 52, preferably ofbrass, at each end thereof. By means of a screw 53 and a leaf spring 54,the top spark gap electrode 5B is connected to an upper terminal cap 55,preferably formed of copper, to which the conductor 42 may be connectedas by soldering; The upper end of the glass housing 45 is provided witha circumferential bead 56 around which the outer periphery of the uppercap 55 may be crimped. The cap 41 is secured on top of the arrester 4!by means of a suitable cement 51, such as asphalt, which completelycovers the upper cap 55 and provides a liquid-tight connectionthereover.

The spark gap assembly 49 is mounted on a brass disc 59 which issurrounded, as illustrated, by means of the inwardly turned edge of ametal disc 60 that may be expanded into engagement with the inner wallof the housing 45. An upper electrode Si is provided which may beconnected by means of a screw 62 to the bottom spark gap electrode 50.The upper electrode 6! may be circular in form and composed of brass.The outer periphery 63 of the upper electrode El may be curveddownwardly, if desired, to tend to con centrate the flow of surgecurrent along the innersurface of the housing 25. While the outerperiphery 63 of the upper electrode Bl has been illustrated as beingcurved downwardly, it will be understood that such construction is notessential, and that satisfactory operation will result if the upperelectrode 6| comprises merely a flat disc with the curved outerperiphery 63 omitted. We have constructed and tested many lightningarresters having the upper electrode 6| formed eesravmeaereacoen as afiat disc only arrd'found that they operate satisfactorily and thatgenerally the surge cur rent will flow along the surface of the glasshous ing 45. I

At the bottom of theglass housing 45 a lower electrode 64 is providedwhich is preferably of forged bronze. The lower electrode 64 is providedwith a flatupper surface and is arranged to be coaxial with the upperelectrode 6!. The lower electrode 64 is provided with a downwardly eX-tending inwardly and externally threaded boss 65 which projects throughan opening 66 in the bottom of the glass'housing 45. A bolt E1 isprovided for engaging the'int'erior threads of the boss 65 to move theterminal assembly 44 upwardly for engaging aground connection between itand the nut 43 which is threaded on the outside of the boss 55 asillustrated. A gasket 68, formed preferably of rubber, is positionedunderneath the lower electrode 64 to provide a seal.

Additional sealing means are provided in the form of a gasket 59 formedpreferably of cork and a brass washer 'H), which are interposed betweenthe undersurface of the housing 45 and the top of the nut 43, as shown.

Interposed between the upper and lower electrodes and 54 is a mass ofvalve material H which may comprise silicon carbide or-other suitablevalve material well known to those skilled .in the art.. Under normaloperating conditions the valve material 'll offers a comparatively highresistance tothe flow of current therethrough. However, on theapplication of high or abnormal voltages due to lightning or switchingsurges, the resistance .-of the valve material H is decreased and surgecurrent is permitted to flow therethrough. It will be observed that themass of valve material TI is positioned in the bottom of the glasshousing in close contact with the inner periphery of the side wallsthereof.

As illustrated more clearly in the diagrammatic representation of thelightning arrester 4| shown in Figure 8 of the drawings, the lightningdischarge in many instances tends to take place along the lines 15between the terminal cap 55 and the upper electrode 61. Thi is due tothe fact that the voltag drop between the cylindrical spark gapelectrodes at the line or upper end is greater than it is near theground or lower end of the arrester, due to the electrostatic capacityof the cylindrical spark gap electrodes 50. That is, the voltage dropacross the spark gap assembly constitutes the major portion of thevoltage drop across the arrester in man instances and, as a result, thearrester may break down or fail between the cap and the upperelectrodeGl.

An illustration of this type of failure is shown at 80 in Figure 9 ofthe drawings. As there indicated, the arrester 4| ha failed along theinner surface of the glass housing 45. Due to the formation of aconducting path by the surge current, causing the failur shown at 80, acomparatively low resistance path for the flow of power current isprovided through the arrester and it therefore permits a slight leak ofpower current. Moreover, the effectiveness of the arrester is destroyedand it should at once be removed. Since the housing 45 i composed of alight-transmitting material, the fault 8B, which emits light raysbecause of flow of follow current, may :be readily noted on a casualinspection from the ground and the location of the faulty arrester willat once be apparent.

In many instances the lightning discharge will take place as indicatedat T6 in Figure 8 of the drawings. The discharge here takes placebetween the upper and lower electrodes 5| and 64 along the inner surfaceof the glass housing 45 through the valve material I I. If the amount ofsurge current is not suflicient to cause the particles of valve materialH to be fused into a conducting path, then on termination of the flow ofsurge current, the arrester will be restored to its normal condition.

A typical example of a failure caused by this type of discharge isillustrated at 8| in Figure 10 of the drawings. As there shown, aportion of the glass housing 45 has been broken away to more clearlyillustrate the conducting path which has been formed by fusion ofparticles of the valve material "into a conducting path by a lightningor switching surge which exceeded the capacity of the arrester 4|. Sincethe particles of valve material 'II are fused together into a conductingpath adjacent the inner surface of the glass housing 45, through which aslight amount of power current can flow from the conductor to ground,the flow of power current along it causes the path to become heated tosuch'an extent that light rays are emitted therefrom. This flow ofcurrent constitutes a leak from the power line and destroys theeffectiveness of the arrester 4!. Moreover, due to the flow of currentacross the spark gap assembly 49, the arrester 4i operates after thefashion of a spark gap radio transmitter, as set forth hereinbefore,thereby causing radio interference in the vicinity of the transmissionline. When an arrester has failed in this manner it should beimmediately removed from the system.

Since the glass housing 45 is relatively thick and we have discoveredthat it should be at least one-eighth inch or greater in thicknessthelight rays emanating from the path 8| are reflected to such an extentthat the discharge is visible from any point around the arrester housing45. Therefore it is unnecessary to make a complete inspection of thearrester 4|, but it is merely necessary to note whether or not a glowappears in the glass housing 4|, either due to the direct rays from thepath 8| heated by the flow of power current or by indirect raysreflected by means of the thick glass housing 45.

Still another form of failure is illustrated at 82 in Figure 11 of thedrawings. As there shown, the flow of surge current has been so great asto cause a partial fusion of the glass housing 45 with a portion of theparticles forming the valve material 1 I. The path 82 thus renderedconducting by the flow of surge current therethrough is capable ofconducting a comparatively large amount of power current, therebydestroying the effectiveness of the arrester 4i and possibly causingconsiderable radio interference.

The typical failures of lightning arresters illustrated in Figures 9, 10and 11 are those which have actually occurred and which we haveobserved. Any arrester that has failed to the extent indicated in any ofthese figures should be immediately removed from the system, since itseffectiveness as an arrester has been destroyed. However, it would notbe possible to detect failures of this type in arresters which areprovided with opaque porcelain housings, as has been the standardpractice in the prior art. It would be necessary either to make adetailed examination of each arrester to determine which one had failedto the extent indicated in these figures ZEST AV'A' I or it would benecessary to await the occurrence of a succeeding lightning or switchingsurge which may be sulficient to completely destroy or ground thearrester to permit a detection of the location of the fault. It is thistype of failure for which our invention is particularl applicable.

It will therefore be apparent that we have along a path which has beenrendered conducting by means of a lightning discharge that causes thepath to be visible from any point around the arrester through the use ofthe glass housing 45 having relatively thick walls. Such visibility is,of course, impossible if a housing of porcelain or other opaque materialis used.

The use of transparent heat resisting material such as Pyrex or thelike, as described before, in the housing 45 results in certainfunctional and operational advantages which arise in part from the useof this transparent material in the walls of the housing 45 and in partfrom the close contact therewith of the valve material II, which hasalso previously been described.

Since certain further changes may be made in the foregoing constructionsand difierent ernbodiments of the invention may be made withoutdeparting from the scope thereof, it is intended that all mattersdisclosed in the foregoing description or shown in the accompanyingdrawings, shall be interpreted as illustrative and not in a limitingsense.

We claim as our invention:

1. In a lightning arrester for an alternating current power transmissionline operating at a power frequency of 60 cycles or less, incombination, a spark gap assembly, a pair of spaced apart electrodes, amass of electric valve material interposed therebetween, said spark gapassembly being connected in series circuit relation with said electrodesand valve material and the combination being disposed to be connectedbetween said line and ground, and a glass housing having relativelythick walls surrounding said spark gap assembly and said valve materialwhich permits light rays emanating from the interior thereof to bevisible from substantially any point around the lightning arrester andwhich permits a visual inspection to determine the existence of anyvariations from the normal condition thereof.

2. The lightning arrester of claim 1 further characterized by the glasshousing being made of a heat resistant type of glass material having avery low coeflicient of expansion and a high melting point.

3. The lightning arrester of claim 1 further characterized by the valvematerial being in substantially physical contact with the housing and bythe transparent glass housing being made of a heat resisting type ofglass material having a very low coeflicient of expansion and a highmelting point.

RALPH H. EARLE. ALWIN G. STEINMAYER.

